Processes for the production of synthetic hydrocarbon fuels and lubricants from synthesis gas, a mixture of hydrogen and carbon monoxide, have been known for some time and, of them, the Fischer-Tropsch process is probably the best known. An account of the development of the process and its more notable characteristics are given in Kirk-Othmer, Encylopedia of Chemical Technology, Third Edition, John Wiley & Sons, New York 1980, Vol. 11, pp. 473-478 in the article on Synthetic Fuels. In the Fischer-Tropsch process, synthesis gas is passed over a catalyst at elevated temperature and pressure to produce a number of carbon monoxide reduction products including hydrocarbons, alcohols, fatty acids and other oxygenated compounds. In favorable circumstances, the oxygenated materials will comprise less than 1 percent of the total liquid product. The hydrocarbon product is highly paraffinic in nature and typically includes hydrocarbon gas, light olefins, gasoline, light and heavy fuel oils and waxy gas oils. Because the higher boiling fractions in the product are generally too waxy for general use either as liquid fuels or lubricants, further processing is normally necessary before they can be used, either as such or by being added to the general pool of products. Upgrading processes of various kinds are described, for example, in U.S. Pat. Nos. 4,125,566, 4,133,841 and 4,234,412 and in these processes a high boiling fraction is subjected to cracking followed by other processes such as hydrotreating in order to remove residual quantities of oxygenated materials and to increase the yield of gasoline boiling range products. U.S. Pat. No. 4,071,574 describes a process for improving the quality of the high boiling fraction, the decant oil being subjected in this case to an initial hydrogenation to saturate olefinics and to remove oxygenates, especially organic acids, after which the higher boiling materials are treated with a catalyst such as ZSM-5 to produce a material with acceptable pour point. A similar process is described in U.S. Pat. No. 4,044,064 in which the hydrotreated Fischer-Tropsch product is fractionated and the kerosone fraction together with the heavy oil fraction boiling above about 345.degree. C. (650.degree. F.) is upgraded using a ZSM-5 type catalyst. In the upgrading process described in U.S. Pat. No. 4,080,397, a mixture of light oil and higher boiling decant oil is first hydrogenated and then fractionated to form a light fraction and a heavy fraction which is subjected to selective cracking. A similar process is described in U.S. Pat. No. 4,044,063. U.S. Pat. No. 4,046,829 describes a process in which the fractionated synthesis product boiling above about 200.degree. C. (400.degree. F.) is first hydrogenated and then treated with a ZSM-5 type catalyst to obtain a product of lower pour point.